Field of the Invention
The invention relates to an apparatus for securing a pressure vessel against over pressure, including a valve assembly being disposed a shaped part (pressurizer dome) joined to the pressure vessel, the valve assembly including at least one safety valve with a valve seat in a housing block.
Such an apparatus is known from German Published, Non-Prosecuted application DE 40 41 418 A1, corresponding to U.S. application Ser. No. 08/080,569, filed Jun. 21, 1993, U.S. Pat. No. 5,398,719. It is used in a primary loop of a pressurized water reactor to keep the pressure in the primary loop below an allowable maximum pressure, of about 170 bar, for instance. To that end, a pressurizer or pressure vessel in a primary loop is connected to a blowoff vessel through a safety valve. In a demand mode or a testing mode, the safety valve is opened through a pilot valve. The actuation of the pilot valve is brought about by the pressure prevailing in the pressure vessel.
In a normal mode, the pressure medium prevails at a valve seat and on a front side of a valve plate of a safety valve at the primary loop pressure of approximately 155 bar and at a primary loop temperature of approximately 350.degree. C. Conversely, downstream of the valve seat and on a rear side of the valve plate, only atmospheric pressure prevails. Due to the resultant temperature difference between the front and back sides of the valve plate, if even only slight gradual leakage occurs, water droplets that bring about local cooling will form on the markedly cooler back side of the valve plate, or in other words downstream of the valve seat. That in turn causes thermal distortion of the valve plate and/or of the valve seat in the housing, so that the initial gradual leak can grow so large that it can no longer be controlled by cooling technology in a blowoff vessel. The result can be that the entire plant has to be shut down.